In the semiconductor industry there is a continuing effort to increase device density by scaling device size. State of the art devices currently have device features with a dimension well below one micron (submicron). To form these features, a photosensitive layer is formed on a substrate or device layer and is exposed to radiation through a reticle. Herein, all such reticles will be referred to as masks. Typically, a mask comprises a substantially transparent base material, e.g. quartz, with an opaque layer having the desired pattern formed thereon as is well known. During device fabrication, the mask is placed over a photosensitive material and incident radiation is provided to the mask. The transparent regions allow the radiation to pass through to the photosensitive material and the opaque regions block out the incident radiation. For one prior art mask design, the opaque regions are made of chrome deposited on a quartz mask. The open regions in the chrome, comprise transparent quartz regions that transmit substantially all the incident radiation.
When fabricating devices at the submicron level, radiation diffraction effects become significant. As a result, portions of the photoresist layer underlying the opaque layer near the edges of the device features are exposed to some diffracted radiation. To minimize the effects of diffraction, phase shifting masks have been used in the prior art.
One type of phase shifting mask employs transparent "phase regions" that transmit some or all of the incident radiation. The phase regions are positioned proximate to open feature regions on the mask which are used to define device features. The phase region shifts the phase of incident radiation approximately 180 degrees relative to zero degree regions, or open feature regions. The radiation transmitted through the 180 degree phase region destructively interferes at the boundaries with radiation from the open feature region, thereby reducing the intensity of radiation incident on the photoresist surface underlying the opaque layer near a feature edge providing good contrast at the boundary of the feature.
In a continuing effort to reduce feature sizes in semiconductor integrated circuits, state of the art semiconductor devices have been requiring smaller and smaller dimensional patterns. The patterns may be formed within a photoresist layer as long as a masked pattern can be resolved within the photoresist layer. The resolution limit which is herein defined as the smallest dimension that can be resolved within the photoresist layer while maintaining an acceptable process window is about: ##EQU1## where k.sub.1 is a "constant" for a given lithographic process (process constant), .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the lens. One skilled in the art appreciates that k.sub.1 is not a true constant but can actually vary. A conventional prior art mask has chrome elements in the opaque regions and openings between the chrome elements. A conventional prior art mask has a k.sub.1 of about 0.8. The resolution limit using a conventional prior art mask is hereinafter called the conventional resolution limit and is about 0.8 .lambda./NA. When .lambda. is about 365 nanometers and NA is about 0.54, the conventional resolution limit is about 0.54 micrometers.
As efforts to reduce semiconductor device sizes have continued, another prior art method that has been employed is the use of attenuated phase shifting masks (APSM). The APSM replaces the opaque layer of prior art masks (which is typically a layer of chrome about 0.1 micrometers thick) with a "leaky" layer which transmits a reduced or attenuated percentage of the radiation incident thereon. For example, a very thin layer of chrome (approximately 300 .ANG.) could be used as the leaky layer. With i-line radiation, or radiation having a wavelength of approximately 365 nanometers, a chrome layer this thin will transmit approximately 10% of the radiation incident on the mask. Additionally, the leaky chrome layer shifts the phase of the transmitted radiation approximately 30.degree. compared to the radiation transmitted through regions of the mask where the leaky chrome layer is not present.
In order to achieve the desired 180.degree. shift, the features are phase shifted an additional 150.degree. degrees, either by etching the mask or by placing a phase shifting material in the open feature regions of the mask. That is, the APSM comprises a layer of leaky chrome covering the entire mask base except for the features which are open regions (i.e. regions having no thin chrome layer) with appropriate phase shifting.
To adjust the phase shift, the phase shifting element must have a known index of refraction. The thickness of the phase shifting element varies depending on the wavelength of the radiation source and the index of refraction of the phase shifting material used. In general, the phase shifting element's thickness is about: ##EQU2## where .lambda. is the wavelength of the radiation and n is the material's index of refraction. Thus, by etching the mask to vary the thickness of the material, the phase shift can be adjusted.
Prior art methods have improved upon the APSM by replacing the leaky chrome layer with films which not only reduce the transmission of radiation but also shift the phase of the radiation transmitted by the desired 180.degree.. Masks employing these improved films which reduce the transmission of radiation as well as shift phase by 180.degree. are referred to as embedded phase shifting masks (EPSM). Materials employed in EPSMs include chrome based materials, as well as films made of molybdenum silicide, titanium nitride and silicon nitride among others.
In FIG. 1A, prior art EPSM 101a is shown. Prior art EPSM 101a is comprised of substantially transparent base material 103a with layer of phase shifting material 105a. Incident radiation 113a is applied to EPSM 101a and is transmitted through the open feature regions 121a of the transparent base material 103a such that open feature 119a of photosensitive layer 115a is exposed to 100% of radiation 113a. Radiation 111a incident upon embedded phase shifting layer 105a is transmitted through EPSM 101a and is applied to regions 117a of photosensitive layer 115a. With a radiation wavelength of 365 nanometers, regions 117a of photo sensitive layer 115a is exposed to approximately 8-12% of radiation 111a is transmitted through phase shifting layer 105a with a phase change of 180.degree. relative to radiation 113a exposed to open feature 119a.
With the prior art EPSM 101a shown in FIG. 1, good contrast is realized at feature boundaries. As a result of the destructive interference which occurs between radiation 111a transmitted through the 180.degree. regions associated with phase shifting layer 105a, and the radiation 113a transmitted through the 0.degree. regions associated with substantially transparent base region 103a, it is appreciated that the destructive interference which occurs between transmitted radiation 111a and 113a provides good contrast at feature boundaries.
It is also appreciated that the 8-12% transmission of radiation 111a through embedded phase shifting layer 105a is not excessive. That is, embedded phase shifting layer 105a is sufficiently "opaque" so as to image and form the desired device patterns on photosensitive layer 115a. If embedded phase shifting layer 105a transmits an excessive amount of radiation 111a, then EPSM 101a would effectively have no "opaque" regions. If, on the other hand, phase shifting layer 105a transmits an inadequate amount of radiation 111a, then the benefits of the destructive interference between the 180.degree. and 0.degree. regions would not be realized. Thus, it is appreciated that phase shifting layers 105a in prior art EPSMs transmit phase shifted radiation in a range of approximately 8-12% to be effective.
It is noted that the 8-12% range described above is merely discussed as an example and that other photosensitive materials may exist which may have optimal lithographic performance when exposed to radiation with EPSMs that transmit phase shifted radiation in a range other than 8-12%. An important point to note is that depending on the photosensitive material used, there exists an optimal percentage of 180.degree. phase shifted radiation transmitted through the EPSM film relative to the corresponding base material to achieve good lithographic performance.
To illustrate better the effects of 180.degree. radiation transmitted through phase shifting layers of prior art EPSMs, FIG. 7 shows the results of simulations comparing critical dimension (CD) versus defocus with varying transmission percentages for 0.25 micrometer contact printing. It is understood that the actual values can vary considerably based upon the feature being formed, exposure parameters, including time and energy of exposure, printer parameters and other factors.
In FIG. 7, curve 701 is a plot representing a CD versus defocus relationship for an EPSM having an 8% transmission. Curve 703 represents an EPSM having a 6% transmission, curve 705 represents an EPSM having a 5% transmission and curve 707 represents a binary mask with an opaque region which transmits 0% radiation. As can be appreciated by those having skill in the art, curve 701 with an 8% radiation transmission exhibits much improved performance as compared to curves 703, 705 and 707. It is also appreciated that by increasing the transmission of radiation from 6% to 8%, a 30% gain in depth of focus is obtained.
With efforts to reduce feature sizes in semiconductor integrated circuits continuing, the resolution limits of semiconductor devices fabricated using radiation wavelengths of 365 nanometers have been reached. As can be seen in Equation 1 above, the resolution limit is directly proportional to radiation wavelength .lambda.. Therefore, increased resolution may be achieved with shorter radiation wavelengths. Accordingly, those skilled in the art have begun efforts to use radiation having shorter wavelengths than i-line radiation, such as deep ultraviolet (DUV) radiation, which has a wavelength of approximately 248 nanometers, for the lithographic fabrication of semiconductor devices. By using DUV with EPSMs, increased resolution should be achieved in accordance with Equation 1 above. However, problems have arisen with prior art EPSMs when using DUV radiation.
As shown in FIG. 7, simulation data clearly indicate that better lithographic performance is obtained with higher radiation transmission through the EPSM layer. Although it is noted that excessive radiation transmission through the EPSM layer may lead to unacceptable resist loss on the wafer level and, in particular, to sidelobe printing, a common problem with attenuated phase shifting mask applications, it is nevertheless desired to have reasonable radiation transmission through the EPSM layer. It is noted that the EPSM film must simultaneously meet a number of requirements such as radiation transmission, 180.degree. phase shifting and chemical resistance. With these limitations in mind, most EPSM films presently under development, for example MoSiON film, are limited to about 6% for 248 nm radiation. Furthermore, the radiation transmittance is expected to be reduced further with radiation wavelengths of 193 nm and shorter due to absorption at shorter wavelengths.
To illustrate, FIG. 1B shows prior art EPSM 101b comprised of substantially transparent base material 103b with embedded phase shifting layer 105b disposed over base material 103b. Radiation 113b having a wavelength equal to 248 nanometers is transmitted through the open feature region 121b of EPSM 101b through transparent base material 103b and is applied to open feature 119b of photosensitive layer 115b at an intensity of 100%. DUV radiation 111b is transmitted through phase shifting layers 105b and is applied to regions 117b of photosensitive layer 115b.
Due to the EPSM film limitations described above, it is noted that a maximum of only 6% of DUV radiation 111b is transmitted through embedded phase shifting layers 105b, unlike the 8-12% transmission of i-line radiation 111a through phase shifting layers 105a in FIG. 1A. Assuming that an 8-12% transmission of radiation is the desired transmission through the EPSM film for the photosensitive material used in this example, the 6% transmission of 180.degree. phase shifted radiation is inadequate to realize the benefits of the destructive interference which should occur at the feature boundaries with 0.degree. phase shifted radiation.
Therefore, what is needed is a method to improve the relative transmission of radiation through EPSM film as compared to the corresponding base material such as a quartz substrate to an optimal value such that adequate lithographic performance is achieved.